Influence of chemical composition and temperature on the passivation of some stainless steels and nickel-based alloys in sulphuric acid

Abstract

The critical current densities for passivation of a number of industrial stainless steels and nickel-based alloys have been measured in 20% H₂SO₄ at temperatures from 30 to 103° C (boiling). The critical current decreases — and then the aptitude for passivation increases — as the chromium and molybdenum contents of the alloys are increased; molybdenum was about three times as effective as chromium. Increasing the temperature extends the range of active dissolution of the alloys and markedly increases the critical current density for passivation. The effect is least severe for the alloys of high Cr and Mo content, but even for these, spontaneous passivation does not take place above 50° C. The logarithm of the active current density varies linearly with the reciprocal of absolute temperature over limited ranges. Apparent activation energies have been measured and proposals for the rate-determining factors in the mechanisms are outlined on the basis of these measurements.